Cytokines as a therapeutic target for allergic diseases: a complex picture

Production of IgE antibodies promotes the development of allergic disorders such as asthma, rhinitis and atopic eczema. Though Th2 cytokines play a pivotal role in the allergic inflammatory cascade, therapeutic strategies which target these factors have not been curative in clinical trials. In humans, the allergic phenotype encompasses a broad spectrum of diverse clinical entities, which are dictated by genetics as well as the nature of the allergen and the time in life at which allergen is encountered. The disparate findings in animal and human systems highlight the complexity of cytokine-mediated events in allergic responses in man. Different allergic phenotypes cannot be distinguished strictly on the basis of Th1 and Th2 cytokines. These observations coupled with an emerging role for regulatory T cells in modulation of the allergic response warrant re-examination of the cytokine network in allergic disease. This article highlights the challenges of dissecting the role of individual cytokines in the development of allergic responses and manifestation of allergic symptoms. Regulatory T cells have been implicated in modulation of the activity of allergen-specific Th1 and Th2 effector cells and immune outcome; however, the characteristics of these regulatory T cells remain largely undefined. The effects of existing and emerging therapies which target Th1, Th2 and regulatory cytokines on established allergic responses are examined in the context of this new paradigm. Taken together, existing data suggest that a multi-faceted approach, which is tailored to each patient, will be required in order to attain clinical benefit.